A method for rendering a signal line that connects a headphone and an audio set wirelessly using an FM wave has been made practical.
In that case, processing to be executed in the audio set, comprises:
(1) forming the sum signal (L+R) and the difference signal (L-R) from the left-channel audio signal L and the right-channel audio signal R; PA1 (2) performing frequency modulation of the sub-carrier signal by the difference signal (L-R), in order to obtain the FM signal S.sub.SUB ; PA1 (3) performing frequency modulation of the main carrier signal by the addition signal S.sub.MPX of the sum signal (L+R) and the FM signal S.sub.SUB ; and PA1 (4) transmitting the FM signal that is obtained in step (3) to the headphone. PA1 sub-carrier frequency f.sub.SUB =50 kHz PA1 main-carrier frequency f.sub.FM =915 MHz. PA1 a first VCO; PA1 a frequency divider circuit for dividing an oscillated signal that is generated by the first VCO; PA1 an oscillation circuit for outputting an oscillation signal having the reference frequency; PA1 a phase comparison circuit for comparing the phases of the divided signal that is outputted from the frequency divider circuit and the oscillated signal of the oscillation circuit; and PA1 a low-pass filter for taking a direct-current voltage having a level that corresponds to the phase difference between the divided signal and the oscillation signal of the oscillation circuit, out of the phase comparison output of the phase comparison circuit, and PA1 the direct-current voltage that is outputted from the low-pass filter is supplied to the first VCO as a control signal for its oscillation frequency, so that a PLL is composed; PA1 the oscillation circuit is composed of a second VCO; PA1 a first audio signal is supplied to the second VCO, and the oscillation frequency is controlled hereby; PA1 a sub-carrier signal is modulated by a second audio signal, so that the modulated signal is formed whose occupied band is outside the band of the first audio signal; and PA1 this modulated signal is added to the direct-current voltage that is outputted from the low-pass filter, and the obtained result is supplied to the first VCO.
That is, frequency multiplexing of the sum signal (L+R) and the difference signal (L-R) into a signal S.sub.MPX is performed, and the multiplexed-frequency signal S.sub.MPX is transmitted.
As an example,
By the way, as a method for performing frequency modulation by the use of PLL, a method is known in which a modulation signal is inputted as a disturbance of the loop, that is, such a method that the modulation signal is added to the output signal of the low-pass filter that is a component of the loop. Therefore, when the FM signal according to the term (3) is to be formed, it is possible to add the addition signal S.sub.MPX to the output signal of the low-pass filter.
However, with modulation by disturbance, it is not possible to obtain sufficient characteristics of distortion factor, frequency response and others, at a lower frequency than the natural frequency .omega.n (which is determined on the basis of the loop gain and the time constant of the low-pass filter). And, in the case where the main-carrier frequency f.sub.FM is as high as 915 MHz as shown above, the frequency-division ratio N of the frequency divider circuit in PLL becomes as large as about 9000, hence, the natural frequency .omega.n becomes 100 Hz or so. Therefore, modulation by disturbance is not suitable for modulation based on the addition signal S.sub.MPX having frequency components lower than 100 Hz such as a musical signal.
Another method is obtain an FM signal by performing frequency modulation of the reference signal of PLL; however, in the case of this method, at a higher frequency than the natural frequency .omega.n, it is impossible to perform modulation having sufficient characteristics, and so it can not be used as the addition signal S.sub.MPX as well.
That is, performing frequency modulation in PLL conflicts with stabilizing the frequency that is the original object of PLL, and so, when modulation is performed in PLL, sufficient characteristics can not be obtained. Therefore, a method of performing modulation in PLL is only practical in mobile telephones, transceivers and other devices in which tone quality is not much of a question.